WASHINGTON – Congress spent months clamoring for the results of special counsel Robert Mueller investigation into Russian interference in the 2016 election, but as they digested his final report on Friday, the only thing that seemed clear was that they want to see more.

House Judiciary Chairman Jerrold Nadler, D-N.Y., issued a subpoena Friday morning for the full report and its supporting evidence. He said he was unsatisfied with the partly blacked-out copy Attorney General William Barr delivered to Congress and the public on Thursday.

Mueller reported across 448 pages about "sweeping and systematic" Russian efforts to benefit Donald Trump in the 2016 election and campaign aides eager for the help, though it ultimately found no conspiracy between Americans and Russians. Mueller also found Trump tried repeatedly to thwart the inquiry, but declined to decide whether that was a crime. Barr and others in the Justice Department later said there wasn't enough evidence for obstruction-of-justice charges.

But Mueller's conclusions were largely confined to whether Trump or others committed federal crimes. Congress has broader options, ranging from holding hearings to amplify the report's findings, passing legislation or potentially even impeaching Trump. House Speaker Nancy Pelosi, D-Calif., said impeachment – the political equivalent of bringing charges against the president – is not yet in the cards.

Democrats plan to organize their response to the report Monday by phone. In announcing the call, Pelosi quoted the Mueller report's conclusion that "Congress has authority to prohibit a president’s corrupt use of his authority in order to protect the integrity of the administration of justice,” which “accords with our constitutional system of checks and balances and the principle that no person is above the law."

Hearings with Barr are planned in early May in the House and Senate Judiciary Committees, and lawmakers have indicated they could summon Mueller to testify too. The House and Senate intelligence committees continue to investigate Russian interference with the election, in an effort to prevent or reduce foreign influence in 2020.

"Congress has an obligation to ensure that activities like those laid out in this report are never repeated," said Sen. Dianne Feinstein of California, the top Democratic on the Judiciary Committee.

Congress could take other steps to reveal more of the information Mueller gathered.

Ned Price, a former special assistant to President Barack Obama at the National Security Council, said committees could document counterintelligence issues that Mueller's investigation exposed, but didn't report on, to educate the American people.

Mueller said FBI personnel embedded with his investigation for more than a year and distributed summaries about foreign intelligence and counterintelligence findings to FBI headquarters and field offices nationwide. Price, a former CIA officer, said that Congress should review that material and synthesize it for publication. He compared the effort to when the Senate Intelligence Committee released a public report in 2014 about CIA rendition and interrogation efforts lawmakers characterized as torture.

“We don’t have a good sense of the documentary history of the exposure, of the risks, of the national security implications about these liaisons with the Russians," said Price, who is now spokesman for the advocacy group National Security Action. “I think it would fill a critical gap that we have in the Mueller report."

The version of Mueller's report released Thursday did not include evidence obtained by grand juries, information the Justice Department thought could harm pending cases, intelligence secrets, or information that could harm the privacy of people who weren't charged. In some places, large passages in the report were blacked-out.

The House subpoena, issued Friday morning, is meant to reveal what is beneath that black ink. Nadler said he also is seeking the evidence Mueller's office gathered.

“I am open to working with the department to reach a reasonable accommodation for access to these materials, however I cannot accept any proposal which leaves most of Congress in the dark, as they grapple with their duties of legislation, oversight and constitutional accountability,” Nadler said.

The demand could meet resistance at the Justice Department and among Republicans in Congress, and could lead to a lengthy court battle. Rep. Doug Collins of Georgia, the top Republican on the Judiciary Committee, said Nadler was grandstanding by demanding millions of records from the investigation in “an impossible timeline” of less than two weeks.

Barr has already offered to let select lawmakers view more of the report as early as Monday, but still not grand-jury evidence. The offer was to lawmakers including top Democrats and Republicans on the Judiciary and Intelligence committees in the House and Senate, along with party leaders from each chamber.

The department will allow lawmakers to take notes, but the notes will remain in a secure facility. “Material redacted in the public version of the report is law enforcement sensitive and confidential; it should not be shared in any form without prior approval of the Department of Justice,” Assistant Attorney General Stephen Boyd told lawmakers in a letter Thursday.

But top Democrats in both chambers rejected the offer Friday as "unworkable" for restrictions against sharing material with other lawmakers and for the lack of grand-jury material. The lawmakers, including Pelosi and Senate Minority Leader Chuck Schumer, D-N.Y., said in a letter to Barr that they would instead press to obtain the full report.

Barr himself is expected to testify about the report May 1 at the Senate Judiciary Committee and May 2 at the House Judiciary Committee. Lawmakers have also asked to hear from Mueller directly.

“I have no objection to Bob Mueller personally testifying,” Barr said Thursday.

House Intelligence Chairman Adam Schiff, D-Calif., said it's "certainly not 'game over.'" He called the Trump campaign’s Russia contacts “unquestionably dishonest, unethical, immoral and unpatriotic,” saying “that is not the subject of vindication,” but of “condemnation." But he hasn't said what action Congress might take next.

“Other actions that may have compromised the president or others around him may or may not be even included in the report,” Schiff said. “In the interest of making sure that our policy is driven by the best interest of the country and not by the personal or financial interests of the president or anyone around him, we need to find out the answers to that.”

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